1. Field of the Invention
The present invention relates to optical storage devices and servo control methods thereof and, more particularly, to an optical storage device and a servo control method that oscillate a lens with respect to an object.
2. Description of the Related Art
In optical storage devices such as a disk device, in order to perform accurate recording/reproducing, focus servo control is performed such that a laser beam is focused on a recording film surface of a disk. In the focus servo control, by feeding back a focus error signal, an objective lens is controlled such that the distance between the objective lens and the disk is maintained to be constant. Recently, with the increase in recording density, the diameter of a laser beam spot is reduced. Thereby, the distance between a disk and an objective lens is reduced. In addition, the focus error signal, which is a linear error signal representing an axial relative position of an objective lens with respect to a disk, functions in the range of ±1 μm or less, which is a very narrow range. Conventionally, in order to orient an objective lens within the range in which feed back control is possible, the signal level of the focus error signal, so-called S curve, has been detected while oscillating the objective lens by open-loop control. In addition, focus entry control has been used in which a focus control loop is closed after it is detected that the position of the objective lens falls within a linear range so as to perform focus control by close-loop control.
FIG. 1 is a block diagram of a disk device. FIGS. 2A, 2B and 2C show conceptual diagrams of a separate optical system and positional relationships between a fixed head and a movable head.
In FIG. 1, a disk device 1 is mainly formed by a control unit 2 and a disk enclosure 3.
The control unit 2 includes a higher interface 11, a buffer memory 12, a MPU 13, an optical disk controller 14, a read/write LSI 15, a DSP 16, a focus error signal (FES) detection circuit 17, a track error signal (TES) detection circuit 18, a zero-cross detection circuit 19, and drivers 20 through 23. In addition, the enclosure 3 includes a laser diode unit (LDU) 31, a detector 32 for ID/MO signal detection, a head amp 33, a spindle motor 34, a magnetic field application part 35, a detector 36a for focus error detection, a detector 36b for track error detection, a lens actuator 37, and a voice coil motor 38.
The separate optical system of FIGS. 2A, 2B and 2C includes a fixed head 201, a movable head 202, a base 203, and a guide rail 204. Additionally, the spindle motor 34 is mounted to the base 203, and a disk 206 is mounted to the spindle motor 34. The movable head 202 includes a movable head carriage 210, a mirror 211, a lens holder 212, an objective lens 213, a focus coil 214 that moves the objective lens in a direction perpendicular to a surface of the disk 206, and a leaf spring 215. Light emitted from the laser diode unit 31 shown in FIG. 1 is guided to the movable head 202 via the fixed head 201. The light reflected by the mirror 211 in the movable head 202 is directed to the disk 206 and focused on the disk 206 via the objective lens 213.
The higher interface 11 performs interfacing with a higher apparatus. Data transmitted to/received from the higher apparatus are temporarily stored in the buffer memory 12. The operation of the device 1 is controlled by the MPU 13 and the disk controller 14.
The read/write LSI 15 performs modulation/demodulation of data. When writing data on a disk, the read/write LSI 15 modulates and supplies write data to the laser diode unit 31, and when reading data from a disk, the read/write LSI 15 controls the laser diode unit 31 such that light for reading is emitted from the laser diode unit 31.
The light emitted from the laser diode unit 31 to the disk 206 via the fixed head 201 shown in FIGS. 2A, 2B and 2C and the movable head 202 is reflected by the disk 206, returned to the fixed head 201 via the movable head 202 shown in FIGS. 2A, 2B and 2C, and supplied to the detector 32 for ID/MO signal detection, the detector 36a for focus error detection, and the detector 36b for track error detection in the fixed head. The detector 32 for ID/MO signal detection detects an ID/MO signal component from the reflected light from the disk 206, and supplies the detected ID/MO signal to the head amp 33. The head amp 33 amplifies and supplies the ID/MO signal to the read/write LSI 15. The read/write LSI 15 demodulates an ID and data from the ID/MO signal. The data demodulated in the read/write LSI 15 are stored in the buffer memory 12.
The detector 36a for focus error detection converts incident light into an electronic signal and supplies it to the focus error signal detection circuit 17. The focus error signal detection circuit 17 generates a focus error signal based on the electronic signal from the detector 36a for focus error detection.
The focus error signal generated in the focus error signal detection circuit 17 is supplied to the DSP 16. The DSP 16 generates and supplies to the driver 22 a focus control signal based on the focus error signal generated in the focus error signal detection circuit 17. Based on the focus control signal from the DSP 16, the driver 22 supplies a driving current to the actuator 37. Based on the driving current from the driver 22, the lens actuator 37 moves the objective lens shown in FIGS. 2A, 2B and 2C in a focus direction, that is, a direction perpendicular to a surface of the disk 206. By moving the objective lens of FIGS. 2A, 2B and 2C in the focus direction, the laser light emitted from the laser diode unit 31 is focused on the disk 206.
In addition, the detector 36b for track error detection converts incident light into an electronic signal and supplies it to the track error signal detection circuit 18. The track error signal detection circuit 18 generates a track error signal based on the signal detected in the detector 36b for track error detection. The track error signal detected in the track error signal detection circuit 18 is supplied to the DSP 16 and the track zero-cross signal detection circuit 19. The track zero-cross signal detection circuit 19 generates and supplies to the DSP 16 a track zero-cross signal based on the track error signal. Based on the track error signal and the track zero-cross signal, the DSP 16 generates and supplies a tracking control signal to the driver 23.
The driver 23 supplies a driving current to the voice coil motor 38 based on the tracking control signal from the DSP 16. The voice coil motor 38 is driven based on the driving current from the driver 23, and moves the movable head 202 in a radial direction of the disk 206 to perform a track following operation.
Further, the MPU 13 generates and supplies a spindle motor control signal to the driver 20. Based on the spindle motor control signal from the MPU 13, the driver 20 rotates the spindle motor 34.
Furthermore, the MPU 13 generates and supplies a magnetic field control signal to the driver 21. Based on the magnetic field control signal from the MPU 13, the driver 21 supplies a driving current to the magnetic field application part 35. The magnetic field application part 35 produces a bias magnetic field corresponding to the driving current from the driver 21. The bias magnetic field produced by the magnetic field application part 35 is applied to the disk 206 and used for recording and/or reproduction of information.
Next, a detailed description is given of the operation of conventional focus entry control.
In an objective lens oscillating operation in the focus entry control, an objective lens is oscillated such that the objective lens surely passes a focus position. In a case where focus entry fails in the oscillating operation, there is a possibility that the objective lens may contact a disk. Thus, in order to prevent this, there is a method of providing a stopper between the disk and the objective lens so as to physically limit displacement of the objective lens. According to the method, even in the case where focus entry fails, the objective lens does not contact the disk, and thus it is possible to prevent data from being damaged.
However, when the focus distance becomes short, in a state where the laser light beam is focused on the disk, the distance between the objective lens and the disk becomes several dozen μm or less. Considering that vertical run-out of a disk due to rotation is several dozen to several hundred μm, it is impossible to provide a stopper for avoiding collision between the objective lens and the disk. Accordingly, in order to avoid collision between the objective lens and the disk, it is necessary to more positively perform focus entry control.
In order to positively perform focus entry control, it is necessary to control the overshoot amount of a focus servo control system after closed-loop control is started by closing the focus servo control system such that the focus error signal falls within a substantially linear range. In order to do so, it is necessary to suppress an error in the relative positions and the relative speeds of the objective lens and the disk immediately before starting closed-loop control by closing the focus servo control system.
However, in the conventional method where focus entry is performed by an oscillation operation of an objective lens, it is impossible to find a relative state between a disk and the objective lens until the S curve of the focus error signal is detected. In a case where, for example, the vertical run-out is ±100 μm in a disk rotated at 1500 rpm, the amplitude of vertical acceleration is ±15.7 mm/s. In order to perform focus entry based on detection of the S curve of the focus error signal, it is necessary to maintain the relative speed to a positive value in an approaching direction. Accordingly, when the objective lens is made to approach the disk at ±16 mm/s, the relative speed with respect to the disk varies in the range of 0.3-31.7 mm/s. Hence, in the case where the maximum value of the relative speed is 31.7 mm/s depending on the timing of vertical run-out of the disk, only 7.89 μs is required to pass the range where the focal depth is 0.25 μm. It is very difficult to perform focus servo control by a DSP whose sampling time for the focus error signal is 10 μs.
Accordingly, in order to realize stable focus entry control, it is necessary to control the relative speed between the disk and the objective lens. In order to do so, a sensor is required that outputs a position signal whose range of focus error detection is wider than that of the focus error signal. For example, in an embodiment shown in Japanese Laid-Open Patent Application No. 11-120569, having the title of the invention “Device and Method for Recording/Reproducing Optical Disk”, a position detection sensor for an objective lens is provided above a focusing actuator of the objective lens. The relative position with respect to a disk is detected by the position detection sensor for the objective lens, thereby aiming to realize stable focus entry. In this embodiment, however, it is necessary to mount the sensor to the actuator, which adversely affects reduction of the size and weight of the actuator.
Additionally, in an embodiment shown in Japanese Laid-Open Patent Application No. 7-287850, having the title of the invention “Optical Pickup Device and Focusing Control Method Thereof”, reflected light from a medium is divided into two beams of light: one is incident on a detection optical system having a low sensitivity to focus error detection and is used for a drawing operation; and the other is incident on a detection optical system having a high sensitivity for focus error detection and is used for focus servo control. According to the method, it is unnecessary to provide a sensor to a movable part. Thus, there is an advantage in that the size of an actuator can be reduced.
Further, the embodiment is characterized in that the direction in which a focus exists is detected by a first detection optical system having a low sensitivity from a position relatively distant from the focus, and an objective lens is moved in the correct direction. There is no description about a method for stably transit to focus servo by a second detection optical system having a high sensitivity. Thus, in an apparatus where the distance between the objective lens and a disk is very narrow, there is a risk the lens and the disk may collide with each other.
Hence, as for a method for outputting a focus error signal having a wide detection range for focus error by improving a conventional photodetector for servo, there is an embodiment shown in Japanese Patent Application No. 2001-93091, having the title of the invention “Optical Device for Recording and Reproducing Information”. With the use of the method, it is possible to detect the relative displacement between an objective lens and a disk from a position distant from a focus position by several dozen μm. Hence, it is possible to perform controlled movement of the objective lens to a focus position at a desired relative speed.
Next, a further detailed description is given of a focus error signal detection method described in Japanese Patent Application No. 2001-93091, with reference to FIGS. 3, 4 and 5.
FIG. 3 is a diagram showing the structure of the fixed optical head 201 shown in FIGS. 2A, 2B and 2C. FIG. 4 is a block diagram of a first focus error signal detection circuit. FIG. 5 is a block diagram of a track error signal and a second focus error signal detection circuit.
The fixed optical head of FIG. 3 is formed by a laser diode 301, a collimate lens 302, beam splitters 303, 304 and 307, a Wollaston prism 305, condenser lenses 306, 308 and 310, a Foucault prism 309, a divided-by-two detector 32, a divided-by-six detector 36, and a divided-by-four detector 36a.
The laser light emitted from the laser diode 301 is emitted from the fixed head 201 of FIGS. 2A, 2B and 2C via the collimate lens 302 and the beam splitter 303, guided to the movable head 202, and directed onto the optical disk 206. The returning light reflected by the optical disk 206 is reflected by the beam splitter 303 and guided to the beam splitter 304. The beam splitter 304 splits the incident light into two light beams and guides the respective light beams to the Wollaston prism 305 and the beam splitter 307. The light incident on the Wollaston prism 305 is focused on the divided-by-two detector 32 via the condenser lens 306, and the ID/MO signal is detected.
On the other hand, the light guided to the beam splitter 307 is divided into two light beams by the beam splitter 307 and the beam splitter 307 guides the respective light beams to the condenser lens 308 and the Foucault prism 309. The light guided to the condenser lens 308 is focused on the divided-by-six detector 36b. The light guided to the Foucault prism 309 is focused on the divided-by-four detector 36a via the condenser lens 310.
FIG. 4 is a block diagram of the first focus error signal detection circuit 17, which is formed mainly by a current-to-voltage (I-V) conversion circuit 401, a first focus error signal (FE1) operation circuit 402, a focus sum (FS) operation circuit 403, and an automatic gain control (AGC) circuit 404. The divided-by-four detector 36a is formed by four detectors F, G, H and I.
According to the Foucault method, each output current of each of the detectors F, G, H and I of the divided-by-four detector 36a, which output current is produced from the returning light focused on the divided-by-four detector 36a, is converted into a voltage signal by the current-to-voltage (I-V) conversion circuit 401. Then, the first focus error signal (FE1) operation circuit 402 subtracts the sum signal of the voltage signals with respect to the detectors G and H from the sum signal of the voltage signals with respect to the detectors F and I, and outputs the resulting difference. On the other hand, the focus sum (FS) operation circuit 403 outputs the sum signal of the voltage signals with respect to the detectors F, G, H and I. Then, the automatic gain control (AGC) circuit 404 divides the output of the first focus error signal (FE1) operation circuit 402 by the output of the focus sum (FS) operation circuit 403 to detect a focus error signal, which is used for normal focus servo.
FIG. 5 is a block diagram of the track error signal and the second focus error signal detection circuit, which is formed by a current-to-voltage (I-V) conversion circuit 501, a track error (TE) operation circuit 502, a second focus error signal (FE2) operation circuit 503, a track sum (TS) operation circuit 504, and automatic gain control (AGC) circuits 505 and 506. The divided-by-six detector 36b is divided into three in the direction orthogonal to a divided-by-two detector for tracking error signal detection according to a conventional push-pull method, and is formed by detectors A1, A2, B1, B2, C1 and C2.
Each output current of each of the detectors A1, A2, B1, B2, C1 and C2 of the divided-by-six detector 36b, which output current is produced from the returning light focused on the divided-by-six detector 36b, is converted into a voltage signal by the current-to-voltage (I-V) conversion circuit 501. The current-to-voltage (I-V) conversion circuit 501 outputs: a voltage signal A (=A1+A2) obtained by converting the output currents of the detectors A1 and A2 into voltages and adding them together; a voltage signal B (=B1+B2) obtained by converting the output currents of the detectors B1 and B2 into voltages and adding them together; a voltage signal C (=C1+C2) obtained by converting the output currents of the detectors C1 and C2 into voltages and adding them together; a voltage signal D (=A1+B1+C1) obtained by converting the output currents of the detectors A1, B1 and C1 into voltages and adding them together; and a voltage signal E (=A2 +B2 +C2) obtained by converting the output currents of the detectors A2, B2 and C2 into voltages and adding them together.
The track error signal (TE) operation circuit 502 calculates a (D−E) signal and outputs a track error signal. The second focus error signal (FE2) operation circuit 503 calculates a (A+B−C) signal, and a focus error signal according to the spot size detection method (hereinafter referred to as the SSD method) is output. A focus error signal according to the SSD method can obtain a second focus error signal having a wider detection area compared to a focus error signal according to the conventional Foucault method, and is suitable for the case where focus control is performed from a position distant from a focus position. Then, the track sum (TS) operation circuit 504 outputs a (D+E) signal.
Then, the automatic gain control (AGC) circuit 505 performs automatic gain control by dividing the track error signal output (D−E) by the sum signal (D+E). In addition, the automatic gain control (AGC) circuit 506 performs automatic gain control by dividing the second focus error signal (A+B−C) by the sum signal (D+E).
FIG. 6A shows an example of the first focus error signal according to the Foucault method, and FIG. 6B shows an example of the second focus error signal according to the SSD method. While the use range of the first focus error signal of FIG. 6A is ±0.25 μm, the second focus error signal of FIG. 6B can be used in the range of −5 to +20 μm.
FIG. 7 shows an operating waveform of focus entry control using the second focus error signal. In FIG. 7, the vertical axis represents the distance between a focus position and an objective lens, and the horizontal axis represents time.
In this embodiment, the objective lens is made to approach a disk by open-loop control in a zone 710, up to the position that is distant from the focus position by 10 μm. Then, when the position of the objective lens is within 10 μm from the focus position, position feedback control according to the second focus error signal is performed. A broken line 701 indicates a control target position signal, and a continuous line 702 indicates the operation of the position feedback control according to the second focus error signal. By approximating the target position signal 701 to zero over time, the position of the objective lens is made close to the focus position by the position feedback control according to the second focus error signal. When the target position signal 701 reaches zero, the input of the position feedback control is switched from the second focus error signal to the first focus error signal. A continuous line 703 indicates the operation of the position feedback control according to the first focus error signal. In the aforementioned manner, it is possible to make the relative position and relative speed of the objective lens with respect to the focus position at the time of switching the first focus error signal to be zero, and thus it is possible to realize stable focus entry control.
However, in the second focus error signal detection according to the SSD method, when the movable head 202 shown in FIGS. 2A, 2B and 2C moves and the total light path length varies, error signal sensitivity and signal level vary. Thus, there is a problem in that the distance from the focus and the signal sensitivity depend on the total light path length.
FIG. 8 shows the characteristics of the second focus error signal at the time when the movable head 202 shown in FIG. 2B is moved by +20 μm (FIG. 2A) and −20 μm (FIG. 2C) and the light path length from the fixed head is changed by 40 mm. In FIG. 8, a line 801 indicates the characteristics of the second focus error signal in the case where the movable head 202 is at the position of FIG. 2A (+20 μm), a line 802 indicates the characteristics of the second focus error signal in the case where the movable head 202 is at the position of FIG. 2B (0 μm), and a line 803 indicates the characteristics of the second focus error signal in the case where the movable head 202 is at the position of FIG. 2C (−20 μm).
Under such characteristics, in the case where the movable head 202 is at the position of FIG. 2B, by switching to the first focus error signal at the time when the level of the second focus error signal becomes zero, it is possible to make the relative position and the relative speed of the objective lens with respect to the focus position at the time of switching the first focus error signal to be zero. Thus, it is possible to realize stable focus entry. However, in the case where, for example, the movable head is at the position of +20 mm shown in FIG. 2A, when the zero level of the second focus error signal is detected, actually, the objective lens exists at the position that is distant from the focus position by −2.5 μm. On this occasion, since the position of the objective lens is outside the linear range of the first focus error signal, it is impossible to stably switch to the first focus error signal.
FIGS. 9A, 9B and 9C show focus entry waveforms on this occasion. FIG. 9A shows the distance between the focus position and the objective lens, FIG. 9B shows the second focus error signal, and FIG. 9C shows the first focus error signal. 901, 902 and 903 of FIGS. 9A, 9B and 9C correspond to the positions of the movable head 202 of FIGS. 2A, 2B and 2C respectively. In the case of FIG. 2B where the movable head position is ±0, even if focus servo control according to the second focus error signal is switched to focus servo control according to the first focus error signal in the vicinity of the time 78.5 ms, the transition is smooth. However, in the case where, for example, FIG. 2A where the position of the movable head is +20 mm, as indicated by the lines 901 in FIGS. 9A, 9B and 9C, though the second focus error signal is zero in the vicinity of the time 78.5 ms, the actual shift of the position of the objective lens from the focus position is −2 μm. Thus, at this time point, since it is outside the linear signal detection range of the first focus error signal, large overshooting occurs at the time of transition to servo control according to the first focus error signal. The same applies to the case of FIG. 2C where the position of the movable head is −20 mm as indicated by the line 903.
In view of the above, it is an object of the present invention to provide an optical storage device and a servo control device therefor that can perform stable focus entry by smoothly performing a transition to focus servo control according to the first focus error signal even in the case where the characteristics of the second focus error signal vary due to a variation in the light path length.